1. Technical Field
The present disclosure relates to data storage unit, more particularly, data storage unit for image compression device.
2. Discussion of the Related Art
The Motion Picture Experts Group (MPEG) has established standards for coding audio-visual information such as movies, video, and music in a digitally compressed format. Thus, motion pictures such as movies can be compressed and transmitted in digitally compressed format.
MPEG-4 is a standard for multimedia for fixed and mobile networks in the digitally compressed format. MPEG-4 also provides an object-based coding where the shape, motion, and the texture of video objects are coded separately. H.263 is a standard for a compression format in transmitting data from video phones through links of non-video phone applications.
In mobile communication networks, MPEG-4 or H.263 standards can be applied to compress images introduced through applications such as digital cameras. Storage of images after compressed coding reduces the amount of memory required to store the images. To replay the images, the compressed data is retrieved and decoded to recover the original images.
As shown in FIG. 1, a conventional technology of motion picture process uses three fields of frame memories: a current frame field (CF) for storing an original image to be compressed; a decoded frame field (DF) for storing a frame decoded from the compressed current frame; a previous frame field for (PF) for storing the decoded frame of prior image. Each frame field includes luminance (Y) and chrominance (Cb/Cr).
As shown in FIG. 2, in the conventional technology, each frame uses 176×144×1.5 pixels to store one image frame structured of YUV 4:2:0 QCIF (Quarter Common Intermediate Format). The field of the luminance component is segmented into nine slices, each of which contains 176×16 pixels.
To obtain a decoded image of a current image, motion estimation is carried out for the current image with reference to a decoded image of a prior image. Then, a decoded image of current image is created by summing the decoded image of prior image with the reconstructed residual image between the current image and the decoded prior image. It is stored in the decoded frame field (DF). A decoded frame of a current frame, e.g. Nth frame, is considered as a previous frame at each sequential step for the next image frame, e.g. (N+1)th frame.
In the conventional technology, each processing step for every input frame uses three frame fields, i.e. CF, DF, and PF. Thus, a three inter-frame compression uses memory capacity of: 114 Kbytes (176×144×1.5×3 pixels) for YUV 4:2:0 QCIF; 456 Kbytes (352×288×1.5×3 pixels) for YUV 4:2:0 CIF; and 1382 Kbytes (640×480×1.5×3 pixels) for YUV 4:2:0 VGA format.
In video devices having large memory capacity, storage of the afore-described frame fields may not pose a significant problem. In mobile video devices such as handheld computers, phones, or PDAs, physical size and power consumption are premium design factors. Reduced memory capacity and power consumption greatly contribute to product appeal.